(1) Field of the Invention
The present invention relates to a semiconductor device used in, for example, optical communication system, particularly to a semiconductor device suitably used in semiconductor optical amplifier (SOA) in which quantum dots are used in active layers.
(2) Description of Related Art
Recently, due to small size and small power consumption, semiconductor optical amplifier and semiconductor laser are spotlighted in the field of optical communication.
Particularly, since the pattern effect is small but gain band is wide, semiconductor optical amplifier, which uses quantum dots as an active layer (hereinafter, referred to as quantum dot SOA), is estimated as a device capable of amplifying the CWDM (Coarse Wavelength Division Multiplexing) signal as a batch.
To put the quantum dot SOA into practical use, in addition to the width of the gain band, the optical characteristics should be independent of the polarization of the optical signals.
With respect to this, it is known that the quantum dots can be formed by utilizing so-called S-K (Stranski-Krastanow) mode growth, which appears at the initial stage of the strain heteroepitaxial growth.
However, generally, in the quantum dots formed in such growth-mode, since the dots have a flat shape and have an anisotropic strain, the gain with respect to the transverse-electric-mode (TE-mode) sensitivity is large, and accordingly, the polarization dependency is large.
Therefore, in order to improve the polarization dependency, for example, the Japanese Patent Laid-Open (Kokai) No.2004-111710 has proposed the following; i.e., barrier layers including quantum dots are repeatedly stuck, and thereby each of the quantum dots are quantum-mechanically combined with each other (refer to paragraph number 0035, 0037, and FIG. 3). Also, the Japanese Patent Laid-Open (Kokai) No. 2004-111710 has proposed the following; i.e., by changing the composition of the barrier layer enclosing the quantum dots, the relationship between the level of light holes and the level of heavy holes in the valence band is controlled; thereby the sensitivity of the quantum dots with the transverse-magnetic-mode (TM-mode) light is controlled to be larger than the sensitivity of the quantum dots with respect to the TE-mode light (refer to paragraph number 0051).